Long Term Evolution-Advanced (LTE-A) is an evolving mobile communication standard which aims to take advantage of 3G mobile communications devices. A feature of LTE-A is carrier aggregation where two or more component carriers are aggregated in order to support wider transmission bandwidths.
It is thought that carrier aggregation will be User Equipment (UE) specific, i.e. each UE in the same cell in a mobile communication network may have different configuration of carrier aggregation.
If the carrier aggregation is symmetric (i.e. same number of downlink component carriers and uplink component carriers), the downlink (DL)/uplink (UL) component carriers are typically associated in pairs in a cell-specific manner. However, in the case of asymmetric carrier aggregation, more than one DL component carriers may be associated in a cell-specific manner with a UL component carrier if there are more DL component carriers than UL component carriers and vice versa.
Once a UE is configured with carrier aggregation, the UE is capable of simultaneously receiving or transmitting on all the component carriers that are aggregated. Thus, the UE may be scheduled over multiple component carriers simultaneously.
Typically, UE is only permitted to transmit on Physical Uplink Shared Channel (PUSCH) on a particular UL component carrier after it has received an UL grant. After the UE has transmitted data on PUSCH on the UL component carrier, ACK/NACK is expected to be transmitted from the associated eNodeB. A problem that arises is which DL component carrier should be used for the PHICH transmission when there are more than one DL component carriers available.
The issue of PHICH carrier linkage is also related to the issue of Physical Downlink Control Channel (PDCCH)-to-PUSCH linkage currently being considered in 3GPP.
There have been attempts at two options to solve the problem of PHICH carrier linkage, namely: (1) DL component carrier for PHICH transmission being the same as that used for the transmission of the UL grant; or (2) DL component carrier for PHICH transmission being linked to the UL component carrier used for PUSCH transmission in a cell-specific manner.